The present invention concerns a new family of chiral doping agents with variable spiral pitch induction, i.e. chemical compounds which, when added in small quantities to a nematic type liquid crystal composition, allow a variable spiral pitch to be induced as a function of isomerisation of part of the chemical structure, for example by means of appropriate irradiation whose wavelength may vary from the ultraviolet (UV) to the visible (VIS), or by means of a heat gradient.
The invention concerns more particularly such chiral doping agents in which the end chains include a polymerisable moiety allowing the spiral pitch induced by isomerisation to be fixed, and thus one or more determined colours to be fixed by using a single liquid crystal composition to which is added a single chiral doping agent according to the invention or a single composition of said chiral doping agents.
In displays operating in reflective mode using chiral nematic phase materials, also designated by the term xe2x80x9ccholestericxe2x80x9d, fixing a colour is known by using:
a purely optical active substance having a chiral nematic phase,
a mixture of optically active substances, all having a chiral nematic phase, or
a substance or a mixture of chiral substances having a nematic phase and containing one or more optically active substances, i.e. chiral doping agents able to be mesomorphic or not and capable of inducing helicity having a determined pitch in the whole mesophase to form a cholesteric phase.
The compounds according to the present invention correspond to the chiral doping agents of the last category and belong more generally to the category of photochemical molecular switches, certain families of which have already been the subject of numerous studies and publications which are cited hereinafter by way of non-exhaustive illustration.
Photoracemisation of binaphtyl and its derivatives, and their helicity induction capacity in the nematic medium have been the subject of in-depth studies in particular by H. J. Deussen and al (Liq. Cryst. 1996, 21, 327; Mat. Res. Soc. Symp. Proc. 1996, 425, 55). It appears however that the photoisomerisation of these compounds involves significant decomposition which is detrimental to the persistence of a determined colour.
B. L. Feringa and al (J. Am. Chem. Soc. 1991, 113, 5468) studied thioxanthene type compounds which isomerise reversibly between two diastereoisomeric forms, with the object of applying this system to the reversible storage of information and thus also allowing application to a colour display. These compounds have the drawback however of having insufficient chemical stability and a switching speed which is much too long (several minutes, or even hours), incompatible for example with a digital display in a timepiece. In the same field, B. L. Feringa""s team also studied derivatives of the dithienylethene type photoisomerisable between an open shape and a closed form (L. N. Lucas and al Chem. Commun. 1998, 2313; Tetrahedron Lett. 1999, 40, 1775).
G. B. Schuster and al (J. Am. Chem. Soc. 1995, 117, 8524) made use of the work of Y. Yokoyama and al (J. Am. Chem. Coc., Com. 1995, 785) on fulgide derivatives to modify the helicity of a cholesteric material. It appears however that a high concentration (xe2x89xa75%) of fulgide derivatives is required to obtain only a moderate change in the spiral pitch (approximately 30%).
H. Hattor and al (Liq. Cryst. 1999, 26, 1085; J. Polym. Sci. 2000, 38, 887) studied spiropyrane derivatives, and more particularly compounds obtained via the co-polymerisation of a cholesterol derivative with different monomers containing a spiropyrane unit activated on positions other than the nitrogen. It appears however that the modification of HTP by UV exposure is insufficient and that modification of the colour reflected by this material (xcex94xcex approximately 10 nm) is insufficient to cover the visible spectrum and envisage making a trichromatic display.
The works of S. N. Yarmolenko and al (Liq. Cryst. 1994, 16, 877) on photomodulable chiral units derived from menthyl and the works of R. P. Lemieux and al (Liq. Cryst. 1996, 20, 741; J. Am. Chem. Soc. 1997, 119, 8111) on photomodulable chiral units derived from thioindigo may also be cited.
The chiral molecular photo-switches, whose features were briefly recalled hereinbefore, practically all integrate the chiral unit and the photoactive unit in a single entity.
The chiral doping agents according to the present invention however include a two-functional chiral unit, at least one of whose functions allows a chemical bond to be established between an isomerisable group and thus allows a separate isomerisable group from the chiral unit to be obtained. In a preferred embodiment, the isomerisable group has a polymerisable or co-polymerisable end chain.
Thus the chiral doping agents according to the invention have three essential structural characteristics by UV or VIS radiation and/or by the addition of photoinitiators:
a biactivated central chiral unit allowing helicity induction in the nematic phase,
at least an isomerisable unit allowing the molecular structure to be varied and thus the helicity to be modulated in the nematic phase, and
polymerisable functions allowing, on the one hand the molecules to be in a way attached by each end and the isomerisation reaction to be blocked at a determined spiral pitch, thus at a determined colour, and on the other hand, via the formation of a gel, preventing the phenomena of diffusion from one pixel to another when one wishes to realise a trichromatic display, as will be explained in the detailed description.
In the following description, xe2x80x9cthe isomerisable unitxe2x80x9d will be named xe2x80x9cphotoisomerisable unitxe2x80x9d assuming that isomerisation is obtained by UV or VIS radiation, the chiral doping agents according to the invention then being named photomodulable chiral doping agents.